There's a great British tradition of cheering on the troops at war, and then treating them abominably when it's over. Heroes today, forgotten tomorrow. Go back to drawings of Crimean veterans, or the mutilated victims of the trenches doing menial jobs to survive, or, more recently, alcoholics in hostels, now unrecognizable as khaki-uniformed "heroes".

Ever since we've had a professional army it has attracted young men, often barely more than boys, who are running from bad homes or a shortage of jobs. They get comradeship, discipline, sometimes bullying, but at any rate a purpose and a structure. Then one day, they have done their time and are pushed back into a civilian world they find alien and aren't equipped to handle.

That's a historic picture, of course, and a lot has changed. Some of it for the better: the Ministry of Defence spends more time and money trying to prepare troops for jobs outside the forces. There are world-class British surgeons and specialists waiting for the most seriously injured cases.

But the bigger changes have been for the worse. The Iraq war, followed by increasing doubts over Afghanistan, means there has not been the 100% public support soldiers might expect. Recent fighting has been nearer to the experience of Suez, when British troops had to do their job in the knowledge that some back home were unsure about the whole thing. That must be hard.

Add to that the hammering the armed forces are taking in the Coalition cuts, with David Cameron himself acknowledging that there will be 11,000 redundancies, possibly including troops now on the frontline in Afghanistan. Some 2,700 are to go from the RAF, where fighter trainees on the verge of getting their "wings" were recently told they weren't needed. The price the navy is paying for its two big new carriers is the lack of enough aircraft to put on them, and 3,300 lost jobs.

That's the background to the announcement on Sunday about giving legal status to the "military covenant". Broadly speaking, the covenant states that in return for being willing to die for their country, members of the armed services have a special right to housing, healthcare, decent pay and education for their children.

Put like that, and who could be against it? Cameron says simply: "we owe them". The Royal British Legion, allied with the News of the World, has fought a powerful and emotive campaign. Jim Murphy, Labour's impressive and plain-speaking defence spokesman, who has campaigned for ages for this, criticises only the fact that the government has had to be pushed into it.

Yet there are some obvious questions. Eagle-eyed readers may have spotted that decent pay, welfare, housing and the rest of it are things that most Britons still living in a welfare state thought they were already supposed to get. Are we now saying that the general level of, for instance, education, is so low that families from the armed forces have to be airlifted out, and given special treatment? Soldiers who have experienced psychological or physical trauma certainly need specialist care. But what are we saying about traumatised victims of other situations, and the wider mental health needs of the country? Are we accepting that good care is now so rare it has to be rationed?

A second issue is that the decisions the coalition has already taken seem to undercut the spirit of the covenant it now wishes to make the law of the land. Making service people redundant hardly suggests they are valued. By changing the measure of inflation used to calculate pensions, including people serving in the forces now, and war widows, ministers are cutting the money they'll get. According to Jim Murphy, a 28-year-old corporal who lost both legs would lose £587,000 by the time he is 70 and the widow of a staff sergeant killed in Afghanistan would lose almost £750,000 during her lifetime.

Will such decisions be open to legal challenge? Will ex-soldiers be able to take the Ministry of Defence to court if they feel they have not been properly prepared for civilian life? The defence secretary, Liam Fox, made clear on Sunday this will not be the case, in which case the whole exercise may be no more than the kind of vapid headline-pleaser and newspaper-tickler that gave New Labour such a bad name.

But I think there is an even bigger question, which is what we actually want our armed services to do. The Iraq war was bad not just for the soldiers but for the politicians who sent them there: the "military covenant" was an act of embarrassed contrition. Today, however brave and militarily successful individual actions are in Afghanistan, hardly anyone believes there can be "victory" there.

One day we will talk to the Taliban. One day, perhaps soon, we will pull back to bases and then start to leave, and the Taliban will return. Maybe the threat of revived terrorist camps and plotting in Afghanistan and northern Pakistan will be damped down by drone attacks. But it will be a long way from the rebuilt, democratised and vaguely liberal Afghanistan the starry-eyed idealists told us would emerge from the gunfire. More soldiers, more gunships, can win more weekly battles. But is successful nation-building really on the table? Is there the political will left in austerity Britain, or debt-challenged Washington?

I doubt it. If so, and if the Taliban return, what are the politicians going to say to the wounded veterans and the widows and orphans of the soldiers who are fighting there now? The question hangs over the military covenant like a pall of grimy smoke.

Today the Commons has a chance to debate "the Middle East, north Africa, Afghanistan and Pakistan" – a huge agenda with many unanswered questions. What's needed is a clear explanation of the future of the Afghan war and the coalition's thoughts on the conditions needed for future engagement.

We are teetering still on the brink of a deeper thrust into the Libyan conflict; but despite enthusiasm at the top of the military and in Whitehall about intensifying efforts there, the chances of state-building in Libya are hardly any higher than in Afghanistan. So how long are we prepared to stay involved in Libya? We are not the world's police force and we cannot afford to pretend we are. We should cancel the Trident replacement and refashion our forces for the more modest and genuinely defensive roles they can play.

Then we should certainly look after them properly, with good equipment, pensions and homes. I fear that a piece of flimsy, leaky legislation aimed at pleasing newspaper editors is no substitute at all.